I guess 3 pounds would be 1% of 300lbs but I think that is really the very rock bottom of what would even be safe to feed. And might be downright UNsafe especially if the horse is very fat and especially for a mini because they can have a serious complication of rapid weight loss, hypertriglyceridemia, that can be fatal. You really have to be careful not to cut their feed back so far that this happens and in my non-veterinary opinion, 1% of the goal weight is too aggressive and I would not do it either! 1% of current weight, maybe, but that's still pretty aggressive - depending on the quality of the hay it may be hard for them to even get sufficient nutrients, calories aside, out of that little. I'm sure your vet knows all this, but, eek. Did he maybe mean 3# in addition to your pasture?
Just for reference, Rowan (~37") is probably ideally about 275# in fighting trim and right now is probably right around 300, losing still but slowly, and he eats (metered by himself, out of 3/4" hole nets) about 5lbs of hay a day in summer and maybe 6-7 in cold weather. So, 1.5-2% depending on how cold it is, of good quality first-cut grass hay, supplements that amount to less than a half cup a day total most of which is timothy pellets, and he normalized his own weight and lost all his fat pads on that once the fresh grass was out of the picture. If he was in regular work it would be just about perfect.
For me, I worry less about the actual weight and more about whether I'm seeing any abnormal fat pads (crest, fat pads on the butt, saddlebags). If you have an even layer all around then that is much less of a concern than these abnormal deposits, which indicate metabolic issues. Obviously it's better still if they are not overweight at all but I'm not going to panic about it if it's "insulation" type fat.
